(1) Field of the Invention
The present invention relates to the field of nuclear medicine. Particularly, the present invention relates to the field of transmission scanning to provide nonuniform attenuation correction within a gamma camera system.
(2) Background of the Invention
Non-uniform photon attenuation is an important factor that affects the qualitative and quantitative accuracy of images collected using Single Photon Emission Computerized Tomography (SPECT) camera systems and can decrease the specificity of these systems for lesion detection. Photon attenuation creates image degradation by interfering with and partially absorbing the radiation emitted from an organ containing a radio-pharmaceutical. Photon attenuation within SPECT systems tends to degrade images by also introducing image artifacts and other distortions that can result in false positive detection or the undetection of lesions. The effects of photon attenuation are especially complex in cardiac studies as a result of nonuniform attenuation attributed to the thorax.
In transmission scanning, the source of radiation is directed toward the associated scintillation detector through the object of interest or patient. If the radiation field is significantly larger than the patient, the radiation source is allowed to directly radiate the detector, causing a high count rate in the scintillation detector. Those parts of the detector that become directly radiated are called unobstructed portions of the detector. It is not advantageous to allow large unobstructed detector areas because the resultant increase in count rate can lead to image degradation and in some cases the event detection electronics and processes can become overloaded (e.g., due to pulse pile-up) and temporarily terminate operation. These high count rates tend to reduce the imaging performance of the imaging system by loading down the signal detection and processing circuitry of the gamma camera.
Transmission computed tomography (TCT) can be used as a method for generating a nonuniform attenuation correction distribution. The transmission image data is gathered using a known source (e.g., line, sheet, or flood) of radiation. If performed separately from the SPECT emission study, the collection of the transmission data requires additional data acquisition time and the collection of the transmission and emission data is susceptible to misregistration effects due to patient (e.g., "object") movement between the data gathering sessions.
In certain SPECT imaging applications, such as with cardiac imaging, it is desirable to image the heart (or other organ) with high resolution image matrix (e.g., having small pixel sizes) because of the small size of the organ. In order to achieve sufficiently small pixel sizes, a small field of view detector is typically used. For example, detectors having a physical field of view of 16".times.16", 15".times.15" and 13".times.13" are used.
However, in order to correct for nonuniform attenuation, as discussed above, a transmission map of the body is acquired. Imaging the whole body requires the full field of view of a large detector (e.g., b 20".times.15"). Therefore, there are problems when performing both SPECT and transmission imaging with a SFOV (small field of view) detector or performing both with a LFOV (large field of view) detector. If the large field of view is used for both transmission and emission, the pixels become too large and image resolution is lost for the emission data.
One solution to this problem is to acquire both the emission and the transmission data using the same small field of view. However, this approach is problematic because in order to determine a proper transmission map to correct the emission data, transmission information regarding the entire body is required--not just the portion of the body imaged in the small field of view. The transmission data becomes truncated. When the body is truncated, artifacts are introduced which must be addressed by complex and cumbersome correction algorithms, which may in themselves be subject to error. In this approach, the transmission data is corrected to account for the fact that the body is larger than the field of view of the detector being used to acquire the transmission image. This correction, called truncation correction, assumes a known shape for the contour of the body and uses this assumption in conjunction with a special algorithm to calculate what the transmission data should have been in those parts of the body outside of the detector's field of view. In effect, this approach attempts to reconstruct a transmission image with an incomplete or truncated set of transmission data.
The above approach has several drawbacks. The data used to generate the transmission map is incomplete and the precision to which the body contour information needs be known is not fully understood or appreciated. This typically can lead to image degradation. In addition, this type of transmission truncation correction requires that the patient be positioned with extreme care, resulting in increased set-up time. Also, the susceptibility of this transmission correction approach to high degrees of image noise is not fully understood or appreciated. Therefore, it would be desirable to gather transmission information that is not truncated in order to improve the nonuniform attenuation corrections factors that are used to correct the emission data. The present invention provides for such advantageous functionality.
Accordingly, it is an object of the present invention to provide more accurate transmission image maps that can be used in conjunction with a large field of view detector with a small field of view detector window. Further, it is an object of the present invention to collect transmission information without requiring image truncation. It is further an object of the present invention to collect emission image data with high resolution. To this extent, it is an object of the present invention to provide the above advantageous elements with a system that collects emission data using a small field of view window but collects transmission image data with a large field of view detector. These and other objects of the invention not specifically recited above will become clear within discussions of the present invention herein.